Manufacturing processes for producing semiconductors frequently use a gas containing a highly reactive gas such as silane gas which, upon contact with air, is deposited to form a solid (SiO.sub.2). FIG. 11 illustrates a typical apparatus for dry etching, for example, Si wafers during the manufacture of semiconductors. An apparatus A mixes the gases H.sub.2, O.sub.2 and CF.sub.4 and feeds the resulting mixture of gases via a tube T to an evacuated process chamber (etching chamber) C, where Si wafers (not shown) are subjected to etching treatment.
In FIG. 11, MFC.sub.1, MFC.sub.2 and MFC.sub.3 are mass flow controllers, V.sub.1, V.sub.2, and V.sub.3 are valves, T is a transport tube, R is a dry pump, V.sub.M is a main valve, Q.sub.1 and Q.sub.2 are traps, and Y is a mechanical booster pump.
The gases H.sub.2, O.sub.2 and CH.sub.4 are fed from sources (not shown) to respective ones of the mass flow controllers MFC.sub.1 -MFC.sub.3 via a respective one of feed lines L.sub.1 -L.sub.3. The output flows of the mass flow controllers are applied to respective ones of the valves V.sub.1 -V.sub.3 and the outputs of the valves are all connected to the tube T. The mass flow rate of CF.sub.4 is normally greater than the mass flow rates of H.sub.2 and O.sub.2. Heretofore, it has been thought that the back flow of CF.sub.4 gas into the H.sub.2 or O.sub.2 valves V.sub.1 and V.sub.2 does not take place because the pressure P of each gas applied to the gas feed lines L.sub.1 and L.sub.2 is at, or greater than, atmospheric pressure while the system (chamber C) into which the mixture of gases is fed is evacuated.
In fact, however, the back flow of the high-flow-rate line gas into the low-flow-rate line gas occurs sometimes in, for example, an apparatus for dry etching a film of silicon oxide. The back flow gas works to form a reaction product in the low-flow-rate gas feed line and the reaction product causes further problems such as leakage in the valve or clogging of the mass flow controller in the low-flow-rate gas feed line. In particular, the problem of back flow becomes significant where the mass flow rate of one of the gases being mixed is extremely low.
To determine what factors affect back flow, a gas mixing and feeding apparatus A was connected to a sampling apparatus S as shown in FIG. 6 and several tests were conducted using test procedures similar to procedures as described later.
The gases He, H.sub.2 and N.sub.2 were applied to lines L.sub.1, L.sub.2 and L.sub.3, respectively. In a first test, pressure regulators P.sub.1 -P.sub.3 and mass flow controllers MFC.sub.1 -MFC.sub.3 were set to provide primary side pressures and mass flow rates as shown in Table I. The purpose of this test was to determine whether He in the high-flow-rate line L.sub.1 (that is, high with respect to line L.sub.3) flows backward into N.sub.2 in the low-flow-rate line L.sub.2.
TABLE I ______________________________________ LINE TYPE OF PRIMARY SIDE FLOW RATE NO. GAS PRESSURE (kgf/cm.sub.2 G) (cc/min) ______________________________________ L.sub.1 He 1 100 L.sub.2 H.sub.g 1 200 L.sub.3 N.sub.g 1 3 ______________________________________
The test confirmed that He in the high-flow rate line L.sub.1, does, in fact, flow backward into the N.sub.2 in the low-flow-rate line L.sub.3. Variation of the gas feed parameters showed that this back flow occurs when the flow rate of N.sub.2 is 5 cc/min. or less in the low-flow-rate line L.sub.3.
It was further found that the maximum level of He of the back flow gas is about 90 ppm when MFC.sub.3 in the low-flow-rate line L.sub.3 was set at its minimum control value of 2.0 cc/min.
Further findings from the results of these tests were:
(1) the back flow becomes stabilized if the gases are introduced for 30 seconds or longer. PA1 (2) the back flow rate is almost the same irrespective of the order in which valve V.sub.1, V.sub.2 and V.sub.3 are operated. PA1 (3) the back flow rate does not change even if the primary side pressures P.sub.1, P.sub.2 and P.sub.3 are changed. PA1 (4) the back flow rate is lower when the pressure at the exit side gas outlet Go at which the different kinds of gases meet in the gas feed line is at vacuum (VAC) than it is at atmospheric pressure (ATM). PA1 (5) the back flow rate becomes lower when the length L between valve V.sub.3 in the low-flow-rate line L.sub.3 and the gas outlet Go is made longer to obtain a larger piping ratio L/D where L is the pipe length and D is the inside diameter of the pipe. PA1 (6) the back flow of He becomes less when the velocity of the N.sub.2 gas is raised by providing an orifice etc. in the low-flow-rate line L.sub.3 at a location upstream of the gas outlet Go. PA1 (7) the back flow rate is changed by changing the orientation of th low-flow-rate line L.sub.3 relative to the gas outlet.